Evidence of defect-promoted reactivity for epoxidation of propylene in titanosilicate (TS-1) catalysts: a DFT study.
Evidence of defect-promoted reactivity for epoxidation of propylene in titanosilicate (TS-1) catalysts: a DFT study.
复制标题
DOI：
10.1021/ja037741v
复制
发表时间：
2004-02
影响因子：
15
通讯作者：
D. H. Wells;W. Delgass;Kendall T. Thomson
中科院分区：
文献类型：
--
作者：
D. H. Wells;W. Delgass;Kendall T. Thomson
我们通过密度泛函理论对不同模型钛硅分子筛（TS - 1）钛中心上的氢过氧基（OOH）中间体进行研究，探究钛位点的微观结构如何影响丙烯环氧化反应活性。研究表明，TS - 1晶格中与硅空位相邻的钛位点比完全配位的钛位点反应活性更高，而我们发现完全配位的钛位点根本不发生反应。我们发现，硅空位附近的丙烯环氧化反应通过一个连续的路径进行：首先，H₂O₂形成氢过氧基中间体Ti - OOH（活化能为15.4千卡/摩尔），然后通过近端氧原子夺取反应与丙烯发生反应（活化能为9.3千卡/摩尔）。这一氧原子夺取步骤因涉及硅空位产生的相邻末端硅醇基团的同时氢化物转移而大大加快。该步骤的过渡态在钛原子上呈现六重氧配位，我们得出结论，与空位相邻的钛原子所处的限制较小的环境，通过允许其松弛到更接近八面体的几何结构，使得过渡态更加稳定。这些结果还表明，与非反应性中间体相比，具有反应活性的氢过氧基中间体通常具有近端氧原子上电子云密度较小以及O - O极化程度较大的特点，这为通过计算筛选具有环氧化反应活性的新型钛硅酸盐结构提供了一种潜在方法。
Our density functional theory study of hydroperoxy (OOH) intermediates on various model titanosilicalite (TS-1) Ti centers explores how microstructural aspects of Ti sites effect propylene epoxidation reactivity and shows that Ti sites located adjacent to Si vacancies in the TS-1 lattice are more reactive than fully coordinated Ti sites, which we find do not react at all. We show that propylene epoxidation near a Si-vacancy occurs through a sequential pathway where H(2)O(2) first forms a hydroperoxy intermediate Ti-OOH (15.4 kcal/mol activation energy) and then reacts with propylene by proximal oxygen abstraction (9.3 kcal/mol activation energy). The abstraction step is greatly facilitated through a simultaneous hydride transfer involving neighboring terminal silanol groups arising from the Si vacancy. The transition state for this step exhibits 6-fold oxygen coordination on Ti, and we conclude that the less constrained environment of Ti adjacent to a vacancy accounts for greater transition state stability by allowing relaxation to a more octahedral geometry. These results also show that the reactive hydroperoxy intermediates are generally characterized by smaller electron populations on the proximal oxygen atom compared to nonreactive intermediates and greater O-O polarization--providing a potential means of computationally screening novel titanosilicate structures for epoxidation reactivity.